The periodic testing policy is considered, for a computer system which fails when the total number of hidden faults exceeds a threshold levelNof tolerence. Periodic tests are scheduled at timeskT(k= 1,2, ⃛) to detect hidden faults. A fault occurs according to a non-homogeneous Poisson processes. When theith fault occurs at ageSi<T, (i) it causes a system failure with probabilityp1(Si), (ii) it becomes a hidden fault with probabilityp2(Si)and is accumulated, or (iii) it is removed with probabilityp3(Si). The expected cost rate is derived. The aim of the paper is to find the optimalTwhich minimizes the expected cost rate per unit time of the policy. Various special cases are considered.